The present invention relates to a reset device for microprocessor, particularly for automotive applications.
Cars are receptacles for an enormous amount of disturbance, much of which originating in the electrical equipment on the car itself, and caused, for example, by on-off operation of various types of inductive loads (ignition coils, injectors, relays) or resistive loads (lights). Nor are they immune to additional external disturbance, such as that caused by radio and television transmitters and repeaters.
As car become more and more sophisticated electronically, such an unfavourable environment cannot be ignored by the designer. In the case of microprocessor systems, in particular, strict control is required of the supply voltage which, usually 5 V, is tapped from the battery voltage by means of a voltage regulator which, regardless of performance, cannot possibly guarantee correct operation under all working conditions.
The purpose behind such control of the supply voltage is to disable operation of the microprocessor in conditions in which it cannot operate reliably. The information for disabling the microprocessor is supplied to it in the form of a reset signal supplied by what is known as a reset device.
At present, reset circuits consist of two comparators and a capacitance, as shown in FIG. 3. The input voltage V.sub.IN from the voltage regulator is supplied to one input of comparator 1, the other input of which is connected to a threshold voltage TH.sub.1 usually close to the nominal value of V.sub.IN (e.g. 4.6 V). Comparator 1 provides for monitoring voltage V.sub.IN and, upon this falling below threshold TH.sub.1, closes switch 2 for grounding condenser 4, which is thus discharged via resistor 3. If voltage V.sub.IN remains below threshold TH.sub.1, after a time depending on time constant RC (where R is the resistance of resistor 3, and C the capacitance of condenser 4), the voltage of condenser 4 falls below a second threshold Th.sub.2, e.g. 4 V, thus switching a second comparator 5, the output of which constitutes the reset signal. Downward switching of the reset signal provides for disabling the microprocessor connected downstream from second comparator 5.
When input voltage V.sub.IN again exceeds threshold Th.sub.1, switch 2 opens and condenser 4 is charged to voltage V.sub.cc by current I supplied by current source 6. After a given time (proportional to Th.sub.2 *C/I, if the condenser is fully discharged), the voltage of condenser 4 gradually exceeds Th.sub.2, and the output of comparator 5 again switches to high for enabling normal operation of the microprocessor. The purpose of the delay in disabling the reset signal is to give the microprocessor time to stabilize its internal circuits upon the supply voltage being restored.
On the above circuit, no indication is given in the event of a brief fall in input voltage not affecting operation of the microprocessor. That is, in the event of input voltage V.sub.IN falling below threshold Th.sub.1, but being restored rapidly, before the voltage of condenser 4, as its is discharged, falls below second threshold Th.sub.2, no switching occurs of second comparator 5, in which case, the voltage of condenser 4 returns to V.sub.cc, and the reset signal remains high, with no interruption in the operation of the microprocessor.
In short, the FIG. 3 circuit:
a) enables the reset signal, if the input voltage falls below threshold Th.sub.1 for longer than a minimum time T.sub.m =RC*1n(V.sub.cc /Th.sub.2); PA1 b) disables the reset signal with a delay of Th.sub.2 *C/I upon V.sub.In again exceeding threshold Th.sub.1 ; PA1 c) is insensitive to events resulting in V.sub.IN falling below first threshold Th.sub.1 for very short periods of time of less than T.sub.m.
The above known circuit presents a number of drawbacks, foremost of which is that the reset signal is enabled solely on the basis of how long the input voltage remains below the first threshold. The capacity of a microprocessor to withstand a fall in voltage, however, depends not only on the duration but also on the amount by which the voltage falls in relation to the nominal value.
A further drawback of the above known circuit is that it lacks precision as regards the delay with which the reset signal is disabled. The condenser, in fact, continues discharging as long as the input voltage V.sub.IN remains below first threshold Th.sub.1. The time required for charging the condenser to second threshold Th.sub.2 obviously depends on the voltage of the condenser when charging commences, which voltage may be zero or anywhere between zero and Th.sub.2, depending on whether the fall in voltage is of long or short duration respectively. This obviously results in imprecision as regards the time required for charging the condenser to threshold Th.sub.2 and disabling the reset signal.
Finally, another drawback of the above known circuit is the slowness with which the condenser is charged by current I. Consequently, in the event of a further fall in input voltage before the condenser has been fully charged, the voltage of the condenser falls as of the value reached by tat time, thus enabling the reset signal in advance. This is especially troublesome in cases where the first fall in input voltage below the first threshold is so short as to keep the condenser above the second threshold (thus failing to enable the reset signal), and a further fall in input voltage occurs before the condenser is fully charged. In this case, the reset signal may be enabled by the sum of the two reductions in input voltage, even though the duration of these, taken singly, would be too short to enable the reset output.